In 2006, it was reported (Takahashi and Yamanaka 2006) that the introduction of genes encoding four protein factors (OCT4 (Octamer-4; POU class 5 homeobox 1), SOX2 (SRY (sex determining region Y)-box 2), KLF4 (Krueppel-like factor 4), and c-MYC) into differentiated mouse somatic cells induced those cells to become pluripotent stem cells, (referred to herein as “induced pluripotent stem cells,” “iPS cells,” or “iPSCs”). Following this original report, pluripotent stem cells were also induced by transforming human somatic cells with genes encoding the similar human protein factors (OCT4, SOX2, KLF4, and c-MYC) (Takahashi et al. 2007), or by transforming human somatic cells with genes encoding human OCT4 and SOX2 factors plus genes encoding two other human factors, NANOG and LIN28 (Lin-28 homolog A) (Yu et al. 2007). All of these methods used retroviruses or lentiviruses to integrate genes encoding the reprogramming factors into the genomes of the transformed cells and the somatic cells were reprogrammed into iPS cells only over a long period of time (e.g., in excess of a week).
The generation iPS cells from differentiated somatic cells offers great promise as a possible means for treating diseases through cell transplantation. The possibility to generate iPS cells from somatic cells from individual patients also may enable development of patient-specific therapies with less risk due to immune rejection. Still further, generation of iPS cells from disease-specific somatic cells offers promise as a means to study and develop drugs to treat specific disease states (Ebert et al. 2009, Lee et al. 2009, Maehr et al. 2009).
Viral delivery of genes encoding protein reprogramming factors (or “iPSC factors”) provides a highly efficient way to make iPS cells from somatic cells, but the integration of exogenous DNA into the genome, whether random or non-random, creates unpredictable outcomes and can ultimately lead to cancer (Nakagawa et al. 2008). New reports show that iPS cells can be created (at lower efficiency) by using other methods that do not require genome integration. For example, repeated transfections of expression plasmids containing genes for OCT4, SOX2, KLF4 and c-MYC into mouse embryonic fibroblasts to generate iPS cells was demonstrated (Okita et al. 2008). Induced pluripotent stem cells were also generated from human somatic cells by introduction of a plasmid that expressed genes encoding human OCT4, SOX2, c-MYC, KLF4, NANOG and LIN28 (Yu et al. 2009). Other successful approaches for generating iPS cells include treating somatic cells with: recombinant protein reprogramming factors (Zhou et al. 2009); non-integrating adenoviruses (Stadtfeld et al. 2008); or piggyBac transposons (Woltjen et al. 2009) to deliver reprogramming factors. Presently, the generation of iPS cells using these non-viral delivery techniques to deliver reprogramming factors is extremely inefficient. Future methods for generating iPS cells for potential clinical applications will need to increase the speed and efficiency of iPS cell formation while maintaining genome integrity.